1. Field of the Invention
This invention relates to a fluid dispenser and particularly, but not exclusively, to a dispenser for dispensing very small quantities of a fluid, such as printing ink, at a rapid rate, on demand. The device is therefore particularly suitable for use in an ink jet printing head.
2. Description of Related Art
FIG. 1 of the accompanying drawings is a schematic cross-sectional view of a known type of fluid dispenser, as used, for example, in drop-on-demand print head. The dispenser 1 includes a fluid supply system 2, comprising a reservoir 3 which is connected to a main chamber 4 of the dispenser via a pipe 5. A drive mechanism 6, which may be a piezo-electric driver, is fitted to one end of the chamber 4, and an outlet jet 7 is provided at the opposite end. The drive mechanism 6 acts as a reciprocating pump. On the outward stroke of the mechanism the fluid, such as printing ink, is drawn into the chamber from the reservoir, and on the forward stroke the fluid is pushed towards the jet 7, so that a drop of fluid is ejected therefrom.
The quality of such a dispenser is determined by the quantity of fluid ejected at each stroke of the drive mechanism, the velocity with which the fluid is ejected, and the ejection time. For a given geometry of the chamber, the pressure at which the fluid is supplied to the chamber and the characteristics of the drive mechanism determine all of those parameters. By increasing the supply pressure and the displacement of the drive mechanism in the forward stroke, either independently or as combined parameters, the ejection quality can be improved. However, if the supply pressure is to be increased above the pressure at the outlet of the jet (which in print heads is generally atmospheric pressure), the fluid column cannot be contained in the chamber during the off periods of the dispenser, i.e. during the periods when no fluid is to be ejected from that particular jet. Fluid will therefore drip out of the jet during those periods.
Hence, the single most influential parameter in achieving high-quality ejection on demand in these known dispensers is the maximum obtainable displacement of the drive mechanism, which is clearly limited.